A trench box or shoring equipment is used in some excavations up to five feet deep and is used in most all excavations over five feet deep, as soil conditions are required under Occupational Safety and Health Administration (“OSHA”) classification. The purpose of the trench box is to prevent the earth walls of a trench, if they should collapse, from falling on workers. A typical trench box has two sides or walls attached to each other with large pipes called spreader bars and openings at either end of the walls. These openings are in place to allow the trench box to stay in the trench, as the excavating equipment removes earth from the front end of the box; the box is pulled forward in the newly excavated trench. The pipe and equipment installation is performed within the walls of the box and the rear opening allows the installed pipe to pass through the box and under the spreader bars as the box is pulled forward. Other types of trench boxes may have three or four walls and are placed in excavations in a single location for the installation of large equipment or structures.
Trench boxes are available in various sizes as needed for the working conditions and to meet OSHA safety requirements. The length of the trench box is chosen based on the length of the pipe or equipment installed. The width is also chosen based on pipe and equipment width and is adjusted by changing the length of the spreader bars. The height of a trench box typically ranges from 4′ to 10′ for a single unit. For excavations requiring additional heights, trench boxes are available in a stacking system to accommodate depths over that of a single unit. The thickness of the trench box walls typically range from four to eight inches and the bottom edges are typically angled to a point at forty five degrees, which helps with anchoring the bottom of the box in the trench and cutting through the earth as it is pulled in the trench.
The typical operation consists of the trench box or other shoring devices placed within a trench excavation. Workers enter the trench box and work within its limits to install the pipe or equipment. As installation is performed, soils may be prepared or replaced to provide a stable foundation for the pipe and equipment. These operations can take place where subsurface ground water is present and in some cases the ground water can be several feet above the bottom of the excavation. This water must be removed to allow for the pipe and equipment installation. The water removal or dewatering is presently accomplished with varying styles of pumps and suction hoses as determined by the type of soil conditions. The installation of these dewatering systems is time consuming which greatly increases the overall installation cost of an underground utility system. The dewatering is also problematic, especially in a hard or rocky type of ground condition. In this condition a deeper excavation is installed within the trench box to accommodate a pump head or suction screen end from a pump hose, to a depth below the working grade of the pipe installation. In many cases, these single depressions and pump ends plug with debris or soil and may not keep up with the amount of ground water that is present, which requires additional pumps and additional labor to monitor the pump ends.
The activity of placing pump ends or suction screens consists of draping a suction hose from a pump over the trench box wall, with its end or suction screen extended to the bottom of the trench. This activity has to be repeated every time the trench box is moved to the next section, which is typically every thirteen to twenty feet and consists of removing the suction hose and moving the pump if needed and replacing the suction hose and setting the suction hose end or screen in a proper location to allow for the installation of the pipe and equipment. In many cases, the amount of time and labor required for moving and setting of pump hoses and monitoring to maintain a safe work environment and proper function of the pumps can exceed the time that is spent in the actual installation of utility pipes or equipment.
The pump hoses hanging in the trench box also creates a safety issue for workers inside of the trench box, as the hanging hoses become an added obstacle to avoid while working and to contend with, when lower and setting pipe and equipment into the trench box. An additional hazard to workers in the trench is created during the raising or lowering of the pump suction hoses in and out of the trench box with the attached suction ends or screens. This hazard is increased with the weight of the pump hoses and the process of removing and resetting them being, repeated several times during the work day.